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Why is perchloric acid called so? I am aware of the rules, but why is it not called hyperchloric acid? Or is it correct to also refer to perchloric acid as hyperchloric acid?

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  • $\begingroup$ I would be more interested why it's even still called like that. IUPAC introduced newer nomenclature ages ago. $\endgroup$
    – Mithoron
    Nov 3, 2020 at 14:51
  • $\begingroup$ We have extra suffix for each of positive oxidation numbers in Czech, so we do not have such problems. Kyselina chlorná HClO, chloritá HClO2, chlorečná HClO3, chloristá HClO4. $\endgroup$
    – Poutnik
    Nov 3, 2020 at 15:13
  • $\begingroup$ In the mid 19th century, chemists apparently named it 'hyperchloric acid' with composition HClO7. But it was later renamed to 'perchloric acid' with a corrected composition $\ce{HClO4}$. So, in common nomenclature, acids with highest O.S is given the per- prefix. There is always IUPAC name to help you in case common name causes confusions. The compound is actually $\ce{ClO3(OH)}$. So, the IUPAC name is " hydroxidotrioxidochlorine". $\endgroup$ Nov 3, 2020 at 16:19
  • $\begingroup$ I doubt anybody would call it hydroxidotrioxidochlorine in nearby future. $\endgroup$
    – Poutnik
    Nov 3, 2020 at 21:40
  • $\begingroup$ @Poutnik. Indeed. That is quite a mouthful. $\endgroup$ Nov 4, 2020 at 3:16

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This nomenclature comes from a long time ago. In the $19$th century, it was decided that the acidic compounds containing H, O and another atom X would be named from the name of this third atom X.

If only one molecule of such an acid was known, the name would be made by adding the ending -ic. Example: Only one acid is known with a Carbon atom, and it is $\ce{H2CO3}$, which is called carbonic acid.

If two acids were known differing in the number of oxygen atoms, the most oxidized would follow the preceding rule. In the least oxidized, the ending would be changed into -ous. Example : In the $19$th century, the two following acids were known containing a nitrogen atom. Well !$~\ce{HNO3}$ was called nitric acid and $\ce{HNO2}$ nitrous acid.

If more than two acids were known differing in the number of oxygen atoms, the chemists would copy the case of the chlorine atom, where four acids were known : $\ce{HClO, HClO2, HClO3, HClO4}$. In this case, the two acids situated in the middle of the list are named according to the previous method : $\ce{HClO2}$ is chlorous acid, and $\ce{HClO3}$ is chloric acid. For the two extreme cases, it was decided to add prefixes, hypo- for the lowest number, and per- for the highest number. As a consequence, $\ce{HClO}$ became hypochlorous acid, and $\ce{HClO4}$ was called perchloric acid.

Today, the same regulations is applied, but the criterion of the number of oxygen atoms has been replaced by the oxidation number. The suffix -ous and -ic must be used if the corresponding acids have a difference of $+2$ in their oxidation numbers.

It should also be mentioned that the previous nomenclature is valid for acids having only one X atom. If an acid have two X atoms, the prefix di- should be added, and tri- for three X atoms. For example, $\ce{H2S2O5}$ is called disulfurous acid, and $\ce{H2Cr2O7}$ is dichromic acid. There are also other prefixes like ortho- and meta- to distinguish between acids having one X and the same oxidation number, but a difference in the number of H atoms.

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    $\begingroup$ In my understanding, the question was not about naming nomenclature the OP is reportedly familiar with. It was rather an etymology question why per- and not hyper-. $\endgroup$
    – Poutnik
    Nov 3, 2020 at 14:59
  • $\begingroup$ Maybe the use of the prefix "per-" is due to the influence of the French language. For example, in French the verb "forer" means "to make a hole in the ground". And if we add the prefix "per-", we get the verb "perforer" which means "to punch, to pierce, to make a hole so as to see the light through it". $\endgroup$
    – Maurice
    Nov 3, 2020 at 17:08
  • $\begingroup$ Thanks for your answer Maurice but like @Poutnik mentioned, my question was why we refer to the acid as perchloric and not hyperchloric acid $\endgroup$
    – m-Xylene
    Nov 5, 2020 at 11:33
  • $\begingroup$ @Metaxylene I guess the answer is similar as for peroxide, where there is also superoxide, but not hyperoxide ( at least not in English, because CZ and DE do have hyperoxide ). $\endgroup$
    – Poutnik
    Nov 5, 2020 at 11:40
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    $\begingroup$ @Metaxylene Is there any confusion ? Naming conventions as for any other language say many "because" to many "whys". They do not always follow straight logic nor optimal way. Note that per- is not a convenience for hyper- but alternative prefix. Also, the pair hypo/hyper prefixes is used for low/high state of the same phenomena, what is not directly applicable here. There is no hypochlorate nor hyper/perchlorite. And last but not least, per- is shorter and that counts. $\endgroup$
    – Poutnik
    Nov 5, 2020 at 11:56

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